1. Field
The teachings in accordance with the exemplary embodiments of this present disclosure generally relate to an apparatus for controlling an induction machine.
2. Background
In general, an induction machine is a motor used in various areas ranging from a field including a fan and a pump to a hoisting field including a crane and an elevator. Meantime, an inverter, which is a device configured to enable a VVVF (Variable Voltage Variable Frequency) operation, can control a torque and speed of a motor using a constant flux operation: V/f) or a vector control.
FIG. 1 is a block diagram illustrating an inverter system according to prior art, where an inverter (2) configured to receive a power from a 3-phase power source (1) drives an induction motor (3) by converting the received power source to a predetermined voltage and frequency. The inverter (2) may be a 2-level inverter or a 3 or more level multilevel inverter, for example.
FIG. 2 is a schematic view illustrating a configuration of an inverter driving system according to prior art, where a constant flux operation method is shown when there is no rotor position sensor for the induction motor (3).
Referring to FIG. 2, a command voltage generation unit (210) serves to determine an inverter output voltage on a synchronous coordinate system using a command frequency, and a conversion unit (220) functions to provide an output of the command voltage generation unit (210) to the inverter (2) by converting the output of the command voltage generation unit (210).
The inverter (2) may apply a 3-phase voltage to the induction motor (3) by receiving a command voltage on a stationary coordinate system, and a conversion unit (240) receives a phase current between the inverter (2) and the induction motor (3) from current detection units (230a, 230b, 230c) and converts the received phase current to d axis and q axis currents on the synchronous coordinate system. An output of the conversion unit (240) may be used as an input for other control of the inverter system.
The command voltage generation unit (210) determines a magnitude of an output voltage in response to a given command frequency. At this time, a pattern of output voltage based on the frequency may be predetermined by a controller (not shown) or defined by a user.
FIG. 3 is an example of output voltage based on frequency for a constant flux operation according to prior art.
That is, the command voltage generation unit (210) may output a command voltage by determining the command voltage from a relationship between a predetermined voltage and frequency as in FIG. 3 in response to the given command frequency. The constant flux operation is to output a voltage corresponding to a frequency required by the predetermined voltage-frequency ratio. However, there is generated a problem where operation of a motor fails when load change is great due to the frequency-voltage relationship or at a field that requires a high torque at a low speed.